(Noob question) preparing the model for sculpting
You have basically two options to go from here but keep in mind that both of them (and sculpting in general) are destructive meaning they're irreversible. So if you like your low-poly mesh it would be smart to make a fallback-copy of it (Shift+D) and put it in a separate collection that you can hide, so it doesn't bother you during sculpting.

With that being said here are your options:


You can Voxel-Remesh your object (the function can be found in the Sculpting tab on the right hand side) to get evenly detailed geometry that you can work with. 
You can use Dyntopo (short for dynamic topology, can be found just above the remeshing tools) which will dynamically add more, finer topology depending on how far you're zoomed in. It takes some time to get used to but it's really fast and handy once you're comfortable with it.